This invention relates generally to laser pump/cavity/amplifier configuration design, and more particularly to realizing single longitudinal mode operation and intracavity frequency conversions for diode-pumped solid-state lasers, as well as optical multipass constructions for pumping laser media and fiber lasers, and for the use of multipass optical amplifier.
Since the so-called xe2x80x9cgreen problemxe2x80x9d was discovered by T. Baer in 1986, it has become well known and has long plagued the stability of the CW intracavity harmonic generation of diode-pumped solid-state (DPSS) lasers. The essential difficulty in solving the xe2x80x9cgreen problemxe2x80x9d results from that, there is a persistent obstacle in effectively obtaining single longitudinal mode CW operation due to the spatial hole-burning effect in solid-state lasers. The related critical design issues are extremely tough. For the past decade, much research has attempted to solve this problem to obtain stable green light. Almost every effort has been made and nearly every way has been tried. However, none of true CW devices or designs has been successful by far with a regular standing-wave cavity. Only ring or very short cavity configurations have been used for this purpose, but they have appreciable inconveniences and limitations.
Baer, many AMOCO scientists and others did primary works and made some detailed reviews to the xe2x80x9cgreen problemxe2x80x9d in their papers and patents, such as the U.S. Pat. No. 5,164,947 (1992) and paper xe2x80x9cIntracavity Doubling of CW Diode-pumped Nd:YAG laser with KTP,xe2x80x9d IEEE J. QE-28, 1148(1992). Baer recognized the xe2x80x9cgreen problemxe2x80x9d and pointed out that there was a fundamental barrier to successful multimode operation of intracavity doubled lasers. (Now, the xe2x80x9cmultimode operationxe2x80x9d should be corrected to be xe2x80x9ca few modes operationxe2x80x9d.) AMOCO and other scientists examined and worked out several important problems, including minimizing spatial hole burning effect with the xe2x80x9ctwisted modexe2x80x9d technology and the various polarization related problems, such as modifying the polarization of the laser modes in the doubling crystal to reduce the likelihood of chaotic amplitude fluctuations.
Controlling spatial hole burning can greatly reduce the possibility of amplitude oscillations. However, weak residual spatial hole burning resulted from imperfect xe2x80x9ctwisted modexe2x80x9d operation can still cause oscillations. In spite of those intense efforts, there remains a determining approach required to achieve dynamically stable single-mode operation with the use of a regular standing-wave cavity when the spatial hole-burning effect is present. What is needed is to provide a powerful form of wavelength selectivity to clamp the peak position of the operating frequency and prevent the laser operation from mode hopping and shifting to wavelengths outside the phase matching curve while controlling appreciable losses to the system.
On the other hand, an etalon within a cavity is commonly used to further control and suppress the harmful mode operation. Etalons typically have the highest spectral mode discrimination. However, the insertion of an etalon often leads to large passive losses and significantly reduces output power. This is especially true, for example, when the etalon is of high-finesse type, or the cavity has a small spatial mode waist and, hence, large beam divergence, and these effects are worse when the etalon is titled. Therefore, as simply inserting an etalon to a laser cavity, these characteristics often lead to the failure of laser operation.
AMOCO scientists realized and considered this key factor and were very close to success. In fact, there was almost one step behind to win the battle of the xe2x80x9cgreen problemnxe2x80x9d. Although they did not cross this decisive step, they have demonstrated several important concerns over the unsolved difficulties inherent in the xe2x80x9cgreen problemxe2x80x9d under the condition of single-mode operation. Following are the major concerns in their paper.
(1) xe2x80x9cThe intracavity harmonic generation laser is much more sensitive to component quality and the associated insertion loss than are most other lasers. To build an efficient intracavity harmonic generation laser, one needs to find some forms of mode selectivity with low loss, which is a significantly difficult task. On the other hand, if enough constraints are placed on the cavity without introducing appreciable losses to the system, stable and efficient operation of intracavity harmonic generation lasers is possible.xe2x80x9d
(2) xe2x80x9cThe doubling efficiency is extremely sensitive to the finesse of the laser cavity so that all these controls must be introduced into the laser cavity without adding appreciable loss to the system.xe2x80x9d
(3) xe2x80x9cThe principle difficulty with this design is that combining a polarizer and a highly birefringent element with a relatively small mode radius (w=100 um) can lead to significant losses. It is found that the green and 1064 nm output from cavities containing Brewster plates are often substantially smaller than those in similar cavities without Brewster plates. A 100 um beam has a far-field divergence angle of 3.4 mr; the off-axis components of the beam are appreciably depolarized by the angle-dependent refractive index. The phase shifts are only a fraction of a wave, but in the presence of a polarizer, these correspond to losses on the order of a fraction of a percent.xe2x80x9d
In conclusion, their major point focuses on that, a relatively small mode waist can lead to significant insertion losses for the intracavity optical elements, particularly for an inserted etalon or Brewster plate in the present case.
In order to overcome the dominant difficulties in the prior art, the present invention offers two solutions for obtaining a dynamically stable single-mode operation with regular standing-wave cavities. These two different methods can be used separately, or collectively at the same time to be more powerful and effective.
(1) Cavity designs with a beam expander are applied to render a large mode waist and an improved beam divergence, so as to significantly reduce the insertion losses for intracavity optical elements, typically for a tilted etalon and Brewster plate; and
(2) A pump head with a thin gain zone is applied to minimize the spatial hole-burning effect. The effect caused by a thin gain region is equivalent to that caused by short cavity configurations in which longitudinal modes are separated substantially, so that the required resolving-power of a frequency-selective form will be largely relaxed, and it becomes possible to use a spectral filter with low insertion losses, such as a birefringent filter or a low-finesse etalon, in realizing single-mode operation.
Further, the thickness of a thin gain zone is considered a critical factor to minimize the spatial hole-burning effect. To construct a thin gain zone at the end of a solid-state laser medium, there are several practical limitations to the commonly-used pumping schemes. The conventional side-pumping schemes are not capable of producing such a thin gain region. On the other hand, the constraints on the end-pumping scheme result from the need of a laser medium with a very large absorption coefficient and a very limited thickness. Therefore, a need still exists in the art to provide an effective method to serve this purpose. Consequently, in the present invention the multipass waveguide pump head has been developed for producing a thin gain zone within a laser medium.
Besides, once successfully solving the xe2x80x9cgreen problemsxe2x80x9d for the intracavity second harmonic generation and obtaining a stable green light output, a new and promising way appears for the development of intracavity third and fourth harmonic generations, which can directly and effectively produce CW UV coherent light from one-single-stage cavity with using two or three nonlinear crystals in a serial manner. This offers a much more attractive solution than the external resonant cavity frequency doubling technology in the art. Nevertheless, such a design used to be considered infeasible in the field.
Further, it is not difficult to produce coherent light over wide spectral ranges from infrared to visible by using the OPO technologies. However, the most OPOs developed for the pulsed mode are not appropriate for CW-mode operations. Intracavity frequency mixing, in contrast to the OPO, is capable of producing coherent light with the CW-mode over wide spectral ranges from infrared to ultraviolet. Based on the technologies developed for frequency doubling, several closely related schemes for frequency mixing and high order harmonic generation are demonstrated for extending the utility of the present invention.
Concurrently, the objects of the present invention are presented as follows.
The major object of the invention is to develop a key technology for overcoming the fundamental barrier to intracavity frequency conversions in DPSS lasers, typically to frequency doubling caused by the so-called xe2x80x9cgreen problemxe2x80x9d, which will result in all solid-state, CW, green, blue or UV light lasers.
Another object of the invention is to find a form of wavelength selectivity with an acceptable low insertion loss in realizing single-mode operation. Such a form typically is a spectral filter incorporated with a beam expander.
Yet, another object of the invention is to use a low resolving-power spectral filter relative to a low frequency-selective loss under the condition of using a pump head with a thin gain zone that leads to minimizing the spatial hole-burning effect, so as to realize single-mode operation.
Consequently, a further object of the invention is to construct a multipass waveguide pump head for producing a thin gain zone, capable of maximizing the absorption and concentrate pumping power within a regular solid-state laser material of a small volume around or less than 1 mm3.
Another object of the invention is to provide a way for frequency doubling in which it is considered unnecessary to keep a nonlinear crystal, such as KTP, as a full- or half-wave plate at the fundamental wavelength with temperature control.
Yet, another object of the invention is to provide a way rather than the xe2x80x9ctwisted modexe2x80x9d technology to minimize the spatial hole burning effect so as to allow using a laser medium with the exhibition of birefringences.
Still another object of the invention is to provide a compact cavity design by inserting a beam expander, which offers a large TEM00 mode volume in the laser rod for mode-matched pumping and a high power intensity in the nonlinear crystal for efficient nonlinear optics actions at the same time, and to achieve a compensation of the thermal lens effect, good beam quality and power scaling.
A further object of the invention is to provide a laser-cavity design with a large TEM00 mode volume so that it becomes possible to use the side-pumping scheme including the use of a corner reflector pump head. The end-pumped laser output power is highly sensitive to the losses as may be introduced by the insertion of intracavity elements such as a quarter-wave plate, which is usually used for producing the xe2x80x9ctwisted modexe2x80x9d operation.
A still further object of the invention is to provide a laser-cavity design to render a large TEM00 mode waist and an improved beam divergence resulting in a great reduction of the insertion loss. This would allow the use of an etalon made from even the green-absorbing material with a relatively large scatter loss. Such an etalon is capable of absorbing the backward-going second harmonic beam so that the intracavity green light feedback effects are minimized.
Yet, another object of the invention is to provide a laser-cavity arrangement to directly and effectively produce a CW UV output from one-single-stage cavity.
Still another object of the invention is to make a laser-cavity arrangement serve as a high-performance intracavity wavelength converter for frequency mixing, which is able to produce true CW coherent light over wide spectral ranges, from infrared to ultraviolet.
Yet, another object of the invention is to provide an approach to center and overlap the bandpasses of several spectral filters and the cavity mode for a hierarchy of frequency-selective set inside a laser cavity so as to stabilize laser operation and maximize the output power.
An additional object of the invention, in accordance with the pump approaches used for the multipass waveguide pump head in producing a thin gain zone, is to develop multipass pumping geometry for pumping laser rods or slabs, or even for pumping optical fiber with rare-earth dopants. The two major approaches are advanced. The first one is characterized by i) the pump cladding which surrounds a much smaller laser material, is designed to have a graded-index or step-index, or ii) using noncircular profile reflector; so that diode bars can directly be coupled for pumping without coupling optics. The second approach is characterized by that the multipass pump head is constructed as an optical waveguide so as to optimize the reflectivity and the pump efficiency.
Another object of the invention is to provide a novel approach in the use of the beam guide input coupler to couple the uncollimated pumping beams from diode bars to a multipass waveguide pump head, resulting in the controlled angles of incidence of the pumping beams so as to accommodate the pumping beams with total-internal-reflection.
Yet, another object of the invention is to provide a novel approach in the use of the beam guide input coupler to couple the collimated pumping beams from diode bars to a multipass waveguide pump head, resulting in the controlled distribution of the angles of incidence of the pumping beam within a small range so as to meet the satisfaction for total-internal-reflection.
Another object of the invention is to provide an approach to protect total-internal-reflection for an optical surface from the so-called xe2x80x9cfrustrated total-internal-reflectionxe2x80x9d when it is contacted with other substances.
Yet, another object of the invention is to provide a novel multipass optical-amplifier design.